Cholera toxin (CT), a virulence factor of Vibrio cholerae, induces an acute diarrheal response in the gut. Two major subunits make up CT, the toxic ADP-ribosylating CTA subunit and the non-toxic, GM1-ganglioside-binding CTB subunit. The CTB subunit consists of a pentameric structure with a molecular mass of approximately 55 kD and is currently used in World Health Organization (WHO)-prequalified oral cholera vaccines due to its capacity to induce CT-neutralizing antibodies. Additionally, CTB is often used as an adjuvant or a molecular scaffold of subunit vaccines because of its strong mucosal immunogenicity.
It is further appreciated that CTB may induce anti-inflammatory and regulatory T cell responses and suppress immunopathological reactions in allergy and autoimmune diseases. For example, the airway administration of CTB ameliorated experimental asthma in a murine model. In a Phase I/II clinical trial, oral administration of CTB, chemically cross-linked to a peptide from the human 60 kDa heat shock protein, blunted uveitis of Behcet's disease. CTB was also shown to blunt the intestinal inflammation of Crohn's disease in mice and humans. These findings indicate the potential of CTB, in addition to its use as a cholera vaccine, as an oral immunotherapeutic agent to blunt intestinal inflammation in Inflammatory Bowel Disease (IBD). However, a comprehensive investigation of CTB's effect on the gastrointestinal (GI) tract has not been done, thus leading to some debate on the protein's usefulness.
In previous studies, a non-glycosylated variant of CTB (CTBp) was rapidly and efficiently manufactured in Nicotiana benthamiana plants. CTBp showed comparable GM1 binding affinity, physicochemical stability and immunogenicity to native (E. coli produced) CTB. Additionally, antibodies elicited by oral administration of CTBp in mice were able to neutralize the cholera holotoxin. These results indicated that CTBp provides a viable alternative to the recombinant protein antigen included in DUKORAL® oral cholera vaccines, potentially facilitating reactive mass vaccination to respond to cholera outbreaks.